Results 1 to 5 of 5
  1. #1
    Join Date
    Aug 2016
    Location
    Ireland
    Posts
    5
    Rep Power
    0

    Effect of altering cooling water flow rate



    Hi

    I'm new to this and am still trying to get my head around some parts. So I was shown an experimental procedure for investigating the performance of a refrigeration plant (R134a) incorporating a water cooled condnser. I varied evaporator loads 1400W, 1200W and 1000W and adjusted the water flow rate across the condenser between 0.01kg/s, 0.02kg/s and 0.03kg/s with results that I interpret that as the flowrate of water across the condenser increases the cooling water differential decreases. But I thought that if Q=mcdt then surely if I increase water flowrate, my temperature differential should increase. What am I missing here? See my recorded results below.

    Thanks and sorry if post is in wrong section

    Barry
    Attached Images Attached Images
    Last edited by scarecrow75; 01-12-2016 at 10:03 PM.



  2. #2
    Join Date
    Oct 2001
    Location
    Nottingham UK
    Posts
    5,061
    Rep Power
    42

    Re: Effect of altering cooling water flow rate

    Hi Barry
    Just think that as the flow rate increases then the water passes through the condenser quicker so it picks up less heat. In effect, with increased flow, we have a shorter 'dwell' time.

    A good example is to picture an electric shower. To get the shower water hotter, you restrict the flow through the heat exchanger - increased dwell time, to make the water cooler, you increase the flow - reduce the dwell time.
    I'm back on the Pale

  3. #3
    Join Date
    Aug 2016
    Location
    Ireland
    Posts
    5
    Rep Power
    0

    Re: Effect of altering cooling water flow rate

    Quote Originally Posted by frank View Post
    Hi Barry
    Just think that as the flow rate increases then the water passes through the condenser quicker so it picks up less heat. In effect, with increased flow, we have a shorter 'dwell' time.

    A good example is to picture an electric shower. To get the shower water hotter, you restrict the flow through the heat exchanger - increased dwell time, to make the water cooler, you increase the flow - reduce the dwell time.
    Thanks Frank. Maybe I'm having a "dumb" moment and I agree with everything above BUT if heat transfer follows the guide of Q(kW) = m(kg/s) x Cp(kJ/kgK) x dt('C) then the increase in flowrate should return an increase in Output?

    Something I'm not grasping right here?

    Any thoughts?

    Barry

  4. #4
    Join Date
    Mar 2010
    Location
    Lisbon
    Posts
    55
    Rep Power
    10

    Re: Effect of altering cooling water flow rate

    Hello Scarecrow75,
    You can increase the flow, decrease Dt and still increase heat transfer, it's only a matter of having a flow increase bigger then the Dt decrease, right?

    Q=m.cp.dt
    example:
    Q1 = 10 x 4 x 10 = 400
    Q2 = 20 x 4 x 8 = 640

    The question is that when you increase the flow you also increase the heat transfer coefficient, so your Dt will be lower but still increasing the total heat transfer.

    CDuque

  5. #5
    Brian_UK's Avatar
    Brian_UK is offline Moderator I am starting to push the Mods: of RE Site Moderator : and general nice guy
    Join Date
    Mar 2001
    Location
    Dorset
    Age
    69
    Posts
    10,384
    Rep Power
    49

    Re: Effect of altering cooling water flow rate

    You are performing your calculation using the heat rejection flow rate when you should be using the heat input side I think.
    Brian - Newton Abbot, Devon, UK
    Retired March 2015
    Please support http://the100project.net/Home

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •